<!DOCTYPE html>
<html>
<head lang="en">
  <meta charset="UTF-8">
  <title></title>
  <style type="text/css">
    * {
      margin: 0;
      padding: 0;
    }
    
    ul {
      list-style: none;
    }
    
    .wrap {
      width: 330px;
      height: 30px;
      margin: 100px auto 0;
      padding-left: 10px;
      background-image: url(imgs/bg.jpg);
    }
    
    .wrap li {
      background-image: url(imgs/libg.jpg);
    }
    
    .wrap > ul > li {
      float: left;
      margin-right: 10px;
      position: relative;
    }
    
    .wrap a {
      display: block;
      height: 30px;
      width: 100px;
      text-decoration: none;
      color: #000;
      line-height: 30px;
      text-align: center;
    }
    
    .wrap li ul {
      position: absolute;
      top: 30px;
      display: none;
    }
  </style>
  <script src="../jquery-1.12.4.js"></script>
  <script>
    $(function () {
      
      //mouseover：鼠标经过事件
      //mouseout:鼠标离开事件
      
      //mouseenter:鼠标进入事件
      //mouseleave：鼠标离开事件
      
      var $li = $(".wrap>ul>li");
      
      //给li注册鼠标经过事件，让自己的ul显示出来
      $li.mouseenter(function () {
        //找到所有的儿子，并且还得是ul
        $(this).children("ul").show();
      });
  
      $li.mouseleave(function () {
        $(this).children("ul").hide();
      });
      
      
    });
  </script>
</head>
<body>
<div class="wrap">
  <ul>
    <li>
      <a href="javascript:void(0);">一级菜单1</a>
      <ul class="ul">
        <li><a href="javascript:void(0);">二级菜单11</a></li>
        <li><a href="javascript:void(0);">二级菜单12</a></li>
        <li><a href="javascript:void(0);">二级菜单13</a></li>
      </ul>
    </li>
    <li>
      <a href="javascript:void(0);">一级菜单2</a>
      <ul>
        <li><a href="javascript:void(0);">二级菜单21</a></li>
        <li><a href="javascript:void(0);">二级菜单22</a></li>
        <li><a href="javascript:void(0);">二级菜单23</a></li>
      </ul>
    </li>
    <li>
      <a href="javascript:void(0);">一级菜单3</a>
      <ul>
        <li><a href="javascript:void(0);">二级菜单31</a></li>
        <li><a href="javascript:void(0);">二级菜单32</a></li>
        <li><a href="javascript:void(0);">二级菜单33</a></li>
      </ul>
    </li>
  </ul>
</div>

</body>
</html>